Recently, research on a wavelength division multiplexing (WDM) communication system for higher-speed and larger-capacity communications has been actively promoted. One of the key optical components used in the WDM communication system is an optical multiplexer/demultiplexer device for coupling or splitting a plurality of lights having respective wavelengths. An exemplary optical multiplexer/demultiplexer device is disclosed in Patent Publication 1 and Non-Patent Publication 1 listed later.
Referring to FIG. 16, a prior art straight-optical-waveguide type optical multiplexer/demultiplexer device shown in FIG. 1 in Patent Publication 1 will be explained. FIG. 16 is a schematic view of a straight-optical-waveguide type optical multiplexer/demultiplexer device. The straight-optical-waveguide type optical multiplexer/demultiplexer device 400 has first and second straight optical waveguides 402, 404 which intersect each other at a junction at an angle 2θ, an optical filter 406 disposed at the junction, and a third straight optical waveguide 408 disposed on a line extended along the first straight waveguide 402 and on the opposite side thereof relative to the optical filter 406. The optical filter 406 is defined by a dielectric multilayer film. Further, the optical filter 406 is positioned and oriented so that an equivalent reflection center plane 406a thereof includes an intersecting point 410 of optical axes 402a, 404a, 408a of the above three straight waveguides 402, 404, 408, and the first straight waveguide 402 defines a mirror image of the second straight waveguide 404 relative to the reflection center plane 406a. 
In Patent Publication 1, although only propagation of two lights having respective wavelengths of 1.3 μm and 1.5 μm is described, propagation of three lights having respective wavelengths can be achieved. For example, in the straight-waveguide type optical multiplexer/demultiplexer device 400 shown in FIG. 16, when the optical filter 406 is an LPF (Long wavelength Pass Filter) allowing a light having a wavelength zone of 1.55 μm to be transmitted therethrough and reflecting lights having respective wavelength zones of 1.49 μm and 1.31 μm, a light having a wavelength of 1.55 μm and input into the first straight waveguide 402 is transmitted through the optical filter 406 to the third straight waveguide 408, and lights having respective wavelengths of 1.49 μm and 1.31 μm are reflected at the optical filter 406 and transmitted to the second straight waveguide 404.
Further, referring to FIG. 17, a multimode-optical-waveguide type optical multiplexer/demultiplexer device employing one optical filter and shown in FIG. 9 in Patent Publication 1 will be explained. FIG. 17 is a schematic view of the multimode-optical-waveguide type optical multiplexer/demultiplexer device. The multimode-optical-waveguide type optical multiplexer/demultiplexer device 420 has first and second multimode-interference type optical waveguide sections 424, 426 disposed on the opposite sides of an optical filter 422 to sandwich it, first and second single-mode optical waveguides 428, 430 connected to the first multimode waveguide section 424, and a third single-mode optical waveguide 432 connected to the second multimode waveguide section 426. The optical filter 422 is made of a dielectric multilayer film reflecting a light having a wavelength of 1.3 μm and allowing a light having a wavelength of 1.5 μm to be transmitted therethrough when each of these lights is input into the optical filter 422 at an incident angle of 0 degree.
In Patent Publication 1, although only propagation of two lights having respective wavelengths of 1.3 μm and 1.5 μm is described, propagation of three lights having respective wavelengths can be achieved. For example, in the multimode-interference type optical multiplexer/demultiplexer device 420 shown in FIG. 17, when the optical filter 422 is an LPF (Long wavelength Pass Filter) allowing a light having a wavelength zone of 1.55 μm to be transmitted therethrough and reflecting lights having respective wavelength zones of 1.49 μm and 1.31 μm, a light having a wavelength of 1.55 μm and input through the first single-mode waveguide 428 into the first multimode waveguide section 424 is transmitted through the optical filter 422 and the second multimode waveguide section 426 to the third single-mode waveguide 432, and lights having respective wavelengths of 1.49 μm and 1.31 μm are reflected at the optical filter 422 and transmitted through the first multimode waveguide section 424 to the second single-mode waveguide 430.
Next, referring to FIG. 18, a rod-lens type optical multiplexer/demultiplexer device disclosed in Non-Patent Publication 1 will be explained. FIG. 18 is a schematic view of a rod-lens type optical multiplexer/demultiplexer device. A rod-lens type optical multiplexer/demultiplexer device 440 has first and second rod lens sections 444, 446 disposed on the opposite sides of an optical filter 442 to sandwich it, first and second optical fibers 448, 450 connected to the first rod lens section 444, and a third optical fiber 452 connected to the second rod lens section 446. The optical filter 442 is made of a dielectric multilayer film. Each of the rod lens sections 444, 446 is provided with a gradient of refractive index inside thereof so that a light beam input into the rod lens sections 444, 446 is collimated, namely, changed into a parallel beam of light, or focused to a point. For example, when a length of the rod lens sections 444, 446 is a quarter of a pitch which is a cycle of a serpentine light path depending on a wavelength of a light, a beam of light input at one end of the rod lens sections 444, 446 is changed into a parallel beam of light at the other end thereof.
When the optical filter 442 is an LPF (Long wavelength Pass Filter) allowing a light having a wavelength zone of 1.55 μm to be transmitted therethrough and reflecting lights having respective wavelength zones of 1.49 μm and 1.31 μm, a light having a wavelength of 1.55 μm and input into the first optical fiber 448 is transmitted through the optical filter 442 to the third optical fiber 452, a light having a wavelength of 1.49 μm and input into the first optical fiber 448 is reflected at the optical filter 442 and transmitted to the second optical fiber 450, and a light having a wavelengths of 1.31 μm and input into the second optical fiber 450 is reflected at the optical filter 442 and transmitted to the first optical fiber 448.
Patent Publication 1: Japanese Patent Laid-open Publication No. 2002-6155 (FIGS. 1 and 3)
Non-Patent Publication 1: Hironori Tanaka et al., Collected Lecture Papers from General Convention of the Institute of Electronics, Information, and Communication Engineers, March, 2004, p. 276, C-3-102; “Development of Optical Multiplexer/demultiplexer Device Having a High Isolation Property”